Dante's broomstick shoulder stretch (A lot of videos available demonstrating it; this is only one; never tried the variation.) Had problems with one shoulder for years - tried chiropractors, prolo therapy, physical therapy - nothing worked very well or at all. One and a half weeks doing the broomstick stretch, all the pain was gone. I've continued with it and the pain has not come back. Work up to 50 reps.
I guess this gives me the opportunity to publicly thank Dante for publicizing it. Wonder why the physical therapist I had at one time didn't know it.
Don't know if it will work in your case or if something more serious is wrong.